Field
This disclosure relates to an image processing method and apparatus.
Description of Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, is neither expressly or impliedly admitted as prior art against the present disclosure.
Depth of field is an optical quantity that refers to the distance between the nearest and farthest objects that appear acceptably sharp to a viewer. This can relate to a person's field of view, an image or video that is captured, or any other imaging system. Depth of field occurs because although a lens is only able to focus on a single point, the loss of clarity increases gradually either side of that single point in the depth direction rather than as a discrete step change between “focussed” and “blurry” or “unfocused”. The “depth” of a point of focus is defined for a viewer as the radial distance from the viewer to the point of focus, such that points of equal depth will form a sphere about the person; the depth of field is also defined along this radial distance either side (in a radial or depth direction) of the point of focus. Depth of field is therefore recognised as an acceptably well focussed region that has boundaries defined by the point at which the loss of focus becomes too great.
This in turn introduces the concept of a stage at which a loss of focus becomes excessive. As mentioned above, precise focus is obtainable only at one radial distance or depth. At that point of focus a point object will produce a point image. At any other radial distance, a point object will produce a blurred spot which will have a size dependent upon how far the point is away from being focused. A threshold may be set for the acceptable size of such a blurred spot such that it is indistinguishable from a point, as seen by the viewer. This threshold may depend upon the manner in which the image is being captured or displayed. For example, an acceptable size of this spot, otherwise known as an acceptable circle of confusion, is established as a practical standard for 35 mm movie images and, separately, for 35 mm stills photography.
But taking these factors into account, in any image capture or display situation, an acceptable circle of confusion can be defined, leading to definable limits on the depth of field in a particular image. An important aspect is the way in which, for real captured images, the depth of field varies according to the aperture size of the arrangement by which the image is captured. Generally speaking, a larger aperture provides a smaller depth of field, and a smaller aperture provides a greater depth of field, other factors being equal. Similarly, the depth of field for a particular point of focus varies with focal length of the imaging system in use, all other factors being equal.
Computer games and films often display images that span a large range of depths, and either as a result of rendering or image capture techniques all of the image may be in focus. Depth is usually conveyed in 2D displays by the use of occlusion and scaling of objects and in 3D images a representation of the apparent 3D depth of each displayed object may also be utilised.
However, when viewing real scenes and objects, a viewer would expect parts of a view to be more blurry relative to the clearer part of the view as this is what a person experiences in day-to-day life. The point of focus of the user is enclosed by region in which the view is relatively sharply focussed, and the size of this region in the depth direction is known as the depth of field as described above.
It is known to render an image with a blurred effect to simulate the depth of field that would be associated with a real object or scene, with methods such as ray tracing. However, these are computationally expensive methods most suitable for images that can be rendered ahead of time which makes them a poor choice for many applications.